The Obama administration sought to intimidate witnesses into not testifying to Congress on Tuesday about whether ATF knowingly allowed weapons, including assault rifles, to be “walked” into Mexico, the chairman of a House committee investigating the program said in an interview Monday.

House Oversight and Government Reform Committee Chairman Darrell E. Issa, California Republican, said at least two scheduled witnesses expected to be asked about a controversial weapons investigation known as “Fast and Furious”received warning letters from the Bureau of Alcohol, Tobacco, Firearms and Explosives to limit their testimony.

Mr. Issa’s committee is set to hear testimony from six current or former ATF employees, including agents and attaches assigned to the bureau’s offices in Mexico, about the operation — in which, federal agents say, they were told to stand down and watch as guns flowed from U.S. dealers in Arizona to violent criminals and drug cartels in Mexico.

The six-term lawmaker aired his concerns about the program in a wide-ranging interview with reporters and editors at The Washington Times on Monday.